This invention relates to the preparation of polyurethane compositions. More particularly, this invention relates to compositions for preparing elastomeric polyurethanes wherein the compositions contain specified difunctional amines as the chain extender.
Elastomeric polyurethanes can be prepared using the so-called "one-shot" method whereby all of the reagents (polyol, amine chain extender, diisocyanate and catalyst) are reacted simultaneously. Alternatively the so-called "two-package" method can be used wherein a first component, an isocyanate-terminated prepolymer obtained by reacting at least a portion of the polyol with the diisocyanate compound, reacted with the second component, an amine chain extender and any remaining polyol in the presence of a suitable catalyst to obtain the final elastomer. The elastomer is believed to be a block copolymer containing urethane and urea groups in the polymer chains.
Up until recently the diamine preferred for use as a chain extender was 4,4'-methylene-bis(2-chloroaniline), which is referred to as Moca. This preference resulted from the excellent mechanical strength properties imparted to urethane elastomers by this diamine. The preference for the use of Moca has been severely restricted since regulations appeared in the Federal Register, Vol. 38, No. 144, July 27, 1973. These regulations classify Moca as a carcinogen and require certain precautions during the manufacture, handling and use of this material. These precautions have encouraged a search for less objectionable chain extending agents without sacrificing the desirable physical properties imparted by Moca. U.S. Pat. No. 3,929,730 discloses that this objective can be achieved by replacing Moca with 1,4-butanediol in combination with an amount of a second chain extender such as phenylene diethanolamine sufficient to solubilize the 1,4-butanediol in the polyol. While such a chain extender mixture may be suitable for use with diphenylmethane-4,4'-diisocyanate (MDI), it does not impart an acceptable level of physical properties, particularly strength and hardness, to elastomers derived from 2,4-tolylene diisocyanate or from mixtures of 2,4- and 2,6-tolylene diisocyanates (TDI).
U.S. Pat. No. 3,920,617 teaches that sulfur-containing diamines of the general formula ##STR1## are particularly suitable chain extending or crosslinking agents for urethane type elastomers, however this class of compounds is only slightly soluble (about 12% by weight, based on polyol) in the conventional types of polyols employed to prepare elastomeric polyurethanes. This level is insufficient to impart the desired physical properties to the elastomer. Surprisingly it has now been found that by including a relatively small amount of certain aprotic solvents the solubility of this class of chain extending agents in high molecular weight polyether- and polyester polyols is more than doubled.